1. Field of Invention
The present invention relates to a drive method and drive device of a vibration actuator having an electromechanical conversion element, and to driving the vibration actuator by generating vibrations in an elastic member by inducing vibration of the electromechanical conversion element.
2. Description of Related Art
FIG. 4 is a perspective view showing one example of a conventional drive device of a vibration actuator. The vibration actuator includes an elastic member 1 formed into a rectangular shape and having force output members 1a and 1b formed on its lower surface. Piezoelectric members 2a and 2b, which are electromechanical conversion elements, are attached to the upper surface of the elastic member 1. The driving force is generated using elliptical movement generated in the force output members 1a and 1b. This is achieved by applying alternating current voltage to the piezoelectric members 2a and 2b and harmonically causing longitudinal vibration and bending vibration in the elastic member 1.
The drive circuit of the vibration actuator includes an oscillator 101, a phase shifter 102, and amplifiers 103 and 104. The oscillator 101 outputs an oscillating alternating current voltage at a fixed frequency. The phase shifter 102 causes the phase of the alternating current of the oscillator 101 to differ by 9.degree.. The amplifier 103 amplifies the output of the phase shifter 102 and applies it to the piezoelectric member 2a. The amplifier 104 amplifies the output of the oscillator 101 and applies it to the piezoelectric member 2b.
When an alternating current voltage having a frequency very close to the inherent frequency of a first-order longitudinal vibration and a fourth-order bending vibration of the elastic member 1 is applied to the vibration actuator in two phases, a harmonic vibration composed of two vibrations is generated so that the device can function as a motor. Force output members 1a and 1b are provided in the parts that become the antinodes of the fourth-order bending vibration, and propulsion (driving force) is generated by an elliptical movement that occurs at the ends of the force output members 1a and 1b.
The structure and load characteristics of such a vibration actuator are disclosed in "Piezoelectric Linear Motors for Application to Driving a Pickup Element," Y. Tomikawa, et al, Proceedings of Fifth Symposium on Dynamics Related to Electromagnetic Force, pp. 393-398, the disclosure of which is incorporated herein by reference.
A movable device is disclosed in "New Ultrasonic Motor," S. Ueha and Y. Tomikawa, Triceps Publication, pp. 145-146, the disclosure of which is incorporated herein by reference.
However, the conventional drive device of a vibration actuator described above has a problem that, when a small number of bursts of oscillating waves (non-continuous waves) are applied as drive signals for micro-movement (small amounts of movement), it moves in the direction opposite to that when continuous waves are applied.
FIG. 5 is a drawing showing the relationship between number of bursts of waves (pulses) and the distance and direction of movement of the drive device of a vibration actuator according to the conventional example.
In the case of this example, when the number of burst waves is from about 10 to about 60 bursts, movement occurs in the direction opposite to that of continuous waves. When the number of burst waves is about 60 bursts or more, movement occurs in the same direction as that of continuous waves. Accordingly, in such a conventional drive method, it was difficult to control micro-movement.
The aim of the present invention is to provide a drive method and drive device of a vibration actuator that facilitates micro-movement using burst signals.